LiFePO4, which is a kind of olivine-type lithium metal phosphate, is not expensive compared to LiCoO2 which has been widely used as a cathode active material for lithium secondary batteries in the related art. Accordingly, LiFePO4 has been expected as a cathode active material of a lithium secondary battery in the future, particularly, a large-scaled lithium secondary battery for vehicles.
As a method of producing LiFePO4, a solid phase synthesis method, a hydrothermal synthesis method, and a sol-gel method are known, but among these, the hydrothermal method capable of obtaining LiFePO4 having a small particle size at a relatively low temperature and in a short time is regarded as the best method. However, LiFePO4 has electronic conductivity lower than that of LiCoO2 of the related art, and thus complexation with a conductive additive has been reviewed.
Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. 2008-270204) discloses a cathode material in which micro carbon fibers are attached to a particle surface of a cathode active material in a netlike shape.
In addition, Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2005-123107) discloses an active material for electrochemical elements. The active material has a structure in which a part or the entirety of a carbon material is coated with olivine-type lithium phosphate.
In addition, Patent Document 3 (Japanese Unexamined Patent Application, First Publication No. 2008-130526) discloses an active material for electrochemical elements. The active material includes primary particles of a lithium-containing iron phosphate compound having an olivine-type structure, and secondary particles having a carbon material. The secondary particles have an approximately spindle shape, or an approximately rhombic shape or elliptical shape in a plan view.
In addition, Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. 2009-43514) discloses a cathode material which includes active material particles composed of an olivine acid salt, a first conductive material attached to a surface of the active material particles, and a second conductive material which is composed of a fibrous carbon material and is coupled with the first conductive material. In the cathode material, each second conductive material is coupled with a plurality of active materials through the first conductive material 158.
In addition, Patent Document 5 (Japanese Unexamined Patent Application, First Publication No. 2008-542979) discloses a cathode material in which a carbon layer coupled to at least a part of a surface of composite oxide particles or a particle assembly by a chemical coupling and/or physical coupling is formed.
In addition, Patent Document 6 (Japanese Unexamined Patent Application, First Publication No. 2009-272041) discloses a cathode material in which fine carbon fibers are attached to a particle surface of a cathode active material in a netlike shape.
In addition, Patent Document 7 (Japanese Unexamined Patent Application, First Publication No. 2007-35488) discloses a cathode material. The cathode material includes a plurality of cathode active material layers, each containing a cathode active material with a different component. A cathode active material having the highest resistance increase rate during overcharge is contained as a main component in at least one layer excluding the outermost surface layer of the positive electrode among the cathode active material layers, and a fibrous carbon material is used as a conductive additive of the layer in which the cathode active material having the highest resistance increase rate is contained as a main component.
In addition, Patent Document 8 (Japanese Patent No. 4192574) discloses an electrode for a nonaqueous electrolyte secondary battery. In the electrode, an electrode mix, which contains an electrode active material composed of a mixed active material of a lithium and manganese composite oxide expressed by a general formula LixMn2-yMyO4 and a lithium composite oxide expressed by a general formula LixM′O2, a conductive material, a fibrous carbon, and a binding agent, is coated on a current collector, and the fibrous carbon is contained in a ratio of 0.2% to 5.0% on the basis of the total amount of the electrode mix.
In addition, Patent Document 9 (U.S. Pat. No. 4,176,804) discloses a production method in which synthesis of LiMPO4 is carried out in the presence of a carbon-containing or electronic conductive material, or a precursor of the electronic conductive material.